The plugin can definitely override the string ‘optional’ in a standard WooCommerce checkout (which is what that looks like).
You’ll need to find the original string in the plugin or theme code. You can check the FAQ (https://www.ads-software.com/plugins/say-what/faq/) or this article (https://plugins.leewillis.co.uk/doc_post/replacing-wordpress-strings-context/) for some examples of what you’re looking for.
Alternatively, the Pro version of the plugin provides a String Discovery mode which means you can search for strings on your site via autocomplete suggestions.
https://plugins.leewillis.co.uk/downloads/say-what-pro
That said, from memory, WooCommerce doesn’t include the brackets in the translatable element, so you’d end up with ‘()’ instead, so I’d suggest looking at an alternate solution – probably hiding with CSS.
